Origin of heavier elements, origin of universe

Origin of heavier elements, origin of universe

Like we said earlier It takes higher and higher temperatures to make larger and larger nuclei fuse together What happens when a star cannot maintain fusion reac<ons? Planetary nebulae Neutron capture (nucleosynthesis paths)

Name of Process Hydrogen burning Helium burning Carbon burning Fuel Products Temperature H He 10,000,000 K He C, O 100,000,000 K C O, Ne, Na, Mg >500,000,000 K Neon burning Ne O, Mg 1,200,000,000 K Oxygen burning Silicon burning O Mg, to S 2,000,000,000 K Mg to S Elements ~ Fe 2,700,000,000 K To 3,500,000,000

Star of ~10 Solar Masses: 12 C à 16 O à 20 Ne à 24 Mg Once carbon is burned, collapse, heating, and about 6 months of 28 Si production - and then about a day of: 28 Si à 32 S à 36 Ar à 40 Ca à 44 Ti à 48 Cr à 52 Fe à 56 Ni Once this sequence is over, the star cannot initiate new fusion reactions by compressing, and it s all over: Gravitational collapse (neutron star, or black hole if massive enough), or Supernova (final collapse involves extreme heating resulting in explosion) 56 Ni decays in a few months to 56 Fe

If you can t make elements heavier than 56 Fe by fusion in stars where did they come from? 62 Ni actually the most stable element, but the pathway to making it involves an uphill step You need extreme condi<ons with lots of extra neutrons released so that something called neutron capture can take place

Ejec<on of elements into space Red giant star is so large that gravita<on is weak for the outermost layers they can be ejected into space (planetary nebulae) Supernova: if core gets too massive and gravity too strong, heat cannot prevent collapse Collapse leads to sudden increase in temperature, outer parts blown away, inner part becomes dense neutron star or black hole

Planetary Nebulae

Where did all these isotopes come from? Add neutrons slowly, they convert to protons before you get the next neutron. This keeps you following the GREEN path (S process). called the valley of stability Some isotopes cannot be made this way R vs. S process! Add neutrons extremely rapidly, they cannot convert to p fast enough and you re following the RED path (neutron rich) Blue arrow shows that come isotopes can only be made this way

S process is the center path isotopes (black) Isotopes to the right of the center path are R process elements (e.g., 87 Rb) Isotopes to the le_ of the center path stem from rapid proton capture or (photo)disintegra<on

A quick look at the universe

The Big Bang 1920 s: Edwin Hubble discovered that all distant galaxies are moving away from us (can tell from Doppler Effect). Velocity is propor<onal to distance!

Velocity = (H 0 )(distance) (H 0 =71 km/sec/mpc) Time = (distance)/(velocity) = 1/H 0 Age of universe is roughly 13.6 x 10 9 years! We cannot tell whether we re near the center or not: center Of course, initial event had monumental heat and density Universe expanding, cooling

Timepoints: 10-34 sec: 10 26 K Nuclear strong force, electroweak force separate 10-7 sec: 10 14 K Protons, an<protons photons 10-4 sec: 10 12 K Number of protons frozen 4 sec: 10 10 K Number of electrons frozen 2 min: 2 H (deuterium) nuclei begin to survive 3 min: Helium nuclei begin to survive 30 min: Frozen 2H, He nuclei (cri<cal predic<on) 300,000 yr: H atoms begin to survive 10 9 yr: Galaxies begin to form

Another predic<on: Abundance of light elements In Big Bang, T and ρ dropping very rapidly with expansion Only about 30 min before T is too low for element forma<on Only have <me for H, He, a bit of Li Amount of 2 H depends on exact density 4 He is stable, most neutrons end up in 4 He Models predict about 25% He, which is close to observa<on

A quick look at our solar system

Planet Sizes

Pamerns in Planets Terrestrial planets are small, dense, rocky. Si, O, Fe, Mg perhaps with Fe cores (H 2 O only a very small frac<on) Jovian planets are large, low density (Saturn is less dense than water), mostly H and He with traces of CH4, NH3, H 2 O they also have lots of moons (many of which have frozen H 2 O).

Inner planets have Higher density the closer to Sun (a_er correc<on for gravita<onal compression) Planet Density Uncompressed Density Mercury 5.44 5.30 Venus 5.24 3.96 Earth 5.50 4.07 Mars 3.36 3.40 (Units in g/cm 3 )

All close to circular All in close to the same plane Orbits: All in the same direc<on More later on meteorites, but the plot shows that the most primi<ve (unaltered by collisions and hea<ng) have the same composi<on as the solar atmosphere